1. Field of the Invention
The invention relates to a high resolution absolute encoder. for transducing mechanical movements into electrical signals for determining position.
2. Description of the Related Art
DE-OS [German Unexamined Published Patent Application] 3,833,115 discloses an optical coder for position measurements in connection with machine tools. This coder includes a light source for emitting coherent, parallel light, a scale provided with grating track having uniform grating constants but a varying ratio of light blocking to light transmitting sections for diffracting the light coming from the light source, and a photoelectric transducer device for receiving a plurality of diffracted light beams of different orders originating from the light beams generated by diffraction in the scale and converts them to electrical signals according to their intensity. The coder further includes a reading device which determines the relationships between the electrical signals emitted by the photoelectric transducer unit on the basis of the patterns of the grating track which change as a result of the relative movement between the scale, on the one hand, and light source and photoelectric transducer unit, on the other hand, and converts these relationships with the aid of a converter device for output as position data for the scale.
EP 0,258,725.A2 further discloses a capacitive linear motion or shaft encoder for controlling and positioning moving objects. In its configuration as shaft encoder, the device is composed of two oppositely disposed spaced stators with a rotor being rotatably arranged in the space therebetween. The rotor is connected so as to rotate together with a shaft that is coupled with the moving object.
Electrically conductive coatings are disposed on the mutually facing sides of the stators and the rotor, so as to form capacitors with the air gap of a capacitance value that changes with the rotation of the rotor.
To realize accuracy and resolution, the coatings are arranged on the rotor and the stators in such a manner that at least two circular ring shaped concentric regions result of which the one region forms the measuring track for a rough measuring system and the other region forms the measuring track for a precision measuring system. The digitally detected values from the rough measuring system are combined with the values of the precision measuring system so that an unequivocally defined absolute value results for the angle of rotation. This is accomplished in that coupling capacitances generated by mutually associated coatings of rotor and stators have a curve shape over the rotation of the shaft which is sinusoidal in sections. The addition of a plurality of mutually displaced capacitance curves having different signs and being sinusoidal in sections results in an approximately purely sinusoidal capacitance curve having a period duration which is a function of the number of measuring tracks.
It has been found that components such as light sources exhibit flows and perform unreliably, particularly under difficult environmental conditions. For example, several observation satellites have become useless due to the failure of the angle encoders in their scanning mechanisms. Most frequently, it was the light sources that malfunctioned although they were provided with backups.
Another critical point are the encoder discs. Usually these are composed of glass and their installation is extremely difficult with respect to preventing vibration and accounting for the thermal behavior of the various materials.